The invention relates to a multiband antenna applicable as an internal antenna in small mobile terminals especially.
In the field of mobile stations, models have become popular which operate in two or more systems, each with a different frequency band. A basic prerequisite for the operation of a communications device is that the radiation and receiving characteristics of its antenna are satisfactory in all bands of the systems in question. Important characteristics are e.g. antenna""s bandwidth and radiation pattern. It is relatively easy to produce a multiband antenna structure with good quality if no limitations are imposed on its size. However, in mobile terminals the antenna should understandably be very compact. Moreover, the current trend is to place the antenna preferably inside the housing of the device for convenience. This makes antenna design more demanding.
An antenna with good enough characteristics which fits inside a small device is in practice most easily implemented as a planar structure: The antenna comprises a radiating plane and a ground plane parallel thereto. To make matching easier, the radiating plane and ground plane are usually interconnected at a suitable point by a short-circuit conductor, producing a so-called planar inverted F antenna, or PIFA. The number of operating bands can be raised to two by dividing the radiating plane by means of a non-conductive slot into two branches, viewed from the feed point, which branches have different lengths so that the resonance frequencies of the antenna portions corresponding to the branches fall into desired points at frequency axis.
Another way to provide a second operating band in a planar antenna is to use a slot radiator. A PIFA structure shown in FIG. 1, disclosed in patent application FI990006, represents such a known antenna. It comprises a ground plane GND and a radiating planar element 120. Connected to the radiating plane is an antenna feed conductor at a point F, and a short-circuit conductor at a point S close to the feed point. The radiating planar element 120 has a slot 130 extending from the edge of the element to the center region thereof. Especially the antenna feed point F is relatively close to the end of the slot 130, which opens into the edge of the plane. The planar element proper resonates in the lower one of the intended operating bands. The dimensions of the slot are such that it resonates in the second, upper, operating band. FIG. 1 also shows a support structure 105 for the radiating plane, being a frame made of dielectric material and having relatively thin walls.
In the dual-band structures described above the upper operating band in particular may prove problematic because of its limited width; its coverage of even a band reserved for a single system may be poor. The problem is emphasized if the aim is to cover the bands of at least two systems, e.g. ones operating in the frequency range 1.7 to 2.0 GHz. Another disadvantage is that the radiation in the horizontal plane especially and in the upper operating band may be less effective than desired. One solution is to increase the number of antenna elements. For example, on top of a radiating plane there may be another radiating plane fed galvanically or electromagnetically. The resonance frequency of the second radiating plane is arranged to be near the upper resonance frequency of the lower plane so that a continuous, relatively wide operating band is provided. Electromagnetically coupled, i.e. parasitic, elements may also be placed on the same geometric plane with the radiating main plane. A disadvantage in the use of parasitic elements is that it adds to the production costs of the antenna and makes it more difficult to achieve repeatability in production. A handicap in the circuit board design of a radio device may be alone a connecting pad required for the short-circuit conductor of a parasitic element on the circuit board below.
An object of the invention is to realize in a new, more advantageous manner an internal antenna for a mobile terminal with at least two operating bands. That which is specified in the independent claim 1 characterizes an antenna structure according to the invention. Some advantageous embodiments of the invention are presented in the dependent claims.
The basic idea of the invention is as follows: The antenna is a PIFA placed inside the housing of a mobile terminal with at least two operating bands. A first resonance falling into a lower operating band is produced by means of a radiating conductive pattern in planar element. To improve characteristics of the antenna in the upper operating band the planar element further comprises a slot according to the invention which goes between the feed point and the short-circuit point of the antenna. The radiator provided by this slot can be considered a quarter-wave slot radiator or a half-wave loop radiator. The PIFA further may have another radiator, which resonates in the upper operation band. An extendable whip element may be added to the structure.
An advantage of the invention is that the upper operating band of an antenna can be widened with the slot or loop radiator according to the invention so that the second band easily covers the bands used by even two mobile communications systems. Another advantage of the invention is that the radiation in the horizontal plane in the upper operating band of the antenna can be made more effective with the loop radiator according to the invention. A further advantage of the invention is that the slot according to the invention can be implemented without substantially degrading the matching in the first operating band of the antenna. A further advantage of the invention is that the structure according to it is simple and inexpensive to fabricate.